1. Field of the Invention
This invention relates to security nuts.
An example of the use of a security nut is in the assembly of security fencing.
2. Description of the related art including information disclosed under 37 CFR 1.97-1.99
In security fencing it is known to use a bolt with a head having a groove in its underside to engage with a fencing post and a nut which comprises a normal hexagonal part and a round dome-shaped body which is attached to the hexagonal part by a neck. The nut is threaded onto the bolt and is tightened by the hexagonal part and when a pre-determined torque is obtained the neck breaks off leaving the dome-shaped part on the bolt. It is almost impossible to remove this part and therefore the arrangement is secure.
However there are three disadvantages of this prior arrangement. Firstly it is usual for the nut and bolt to be galvanized and when the hexagonal part of the nut breaks off the dome-shaped part this leaves a face which has not been galvanized and which may go rusty.
Secondly, sometimes the pre-determined torque to separate the two parts of the nut occurs before the nut is fully tightened and then the dome-shaped part of the nut is left on the bolt in a slack condition which probably means that the bolt has to be cut off and a new nut and bolt assembly inserted.
Thirdly, since the hexagonal part of the nut is scrapped, the nut is expensive.
It is also known, e.g. from European Patent Application No 0 013 274, to have a cylindrical nut with formations on its end face which allow the nut to be tightened with a tool but not slackened. Disadvantages of this construction are firstly, that it is difficult to align the tightening tool with the nut and, secondly, due to the cylindrical shape of the nut, it is possible to rotate and slacken the nut with a pair of grips which can engage the cylindrical surface. The nut therefore is not secure.
Amongst the art cited against European patent Application No 0 013 274 is French Patent No 1,232,468 which discloses a bolt with a cylindrical head having formations on its end which may be engaged with a socket to tighten the bolt but which prevent the bolt being slackened. This overcomes the first of the disadvantages of European Patent Application No 0 013 274 but not the second. The cylindrical head may be engaged with grips and slackened so that it is not secure.
It is an object of the present invention to provide a security nut which overcomes the above disadvantages.